Calomel

Hooper says-

“This name, which means `beautiful black,' was originally given to
the Æthiop's mineral, or black sulphuret of mercury. It was afterwards
applied in joke by Sir Theodore Mayerne to the chloride of mercury, in
honour of a favourite negro servant whom he employed to prepare it. As
calomel is a white powder, the name is merely a jocular
misnomer.” —Medical Dictionary.